Posttranslational modification of eukaryotic proteins, particularly therapeutic proteins such as immunoglobulins, is often necessary for proper protein folding and function. Because standard prokaryotic expression systems lack the proper machinery necessary for such modifications, alternative expression systems have to be used in production of these therapeutic proteins. Even where eukaryotic proteins do not have posttranslational modifications, prokaryotic expression systems often lack necessary chaperone proteins required for proper folding. Yeast and fungi are attractive options for expressing proteins as they can be easily grown at a large scale in simple media, which allows low production costs, and yeast and fungi have posttranslational machinery and chaperones that perform similar functions as found in mammalian cells. Moreover, tools are available to manipulate the relatively simple genetic makeup of yeast and fungal cells as well as more complex eukaryotic cells such as mammalian or insect cells (De Pourcq et al., Appl Microbiol Biotechnol, 87(5):1617-31). Despite these advantages, many therapeutic proteins are still being produced in mammalian cells, which produce therapeutic proteins with posttranslational modifications most resembling the native human proteins, whereas the posttranslational modifications naturally produced by yeast and fungi often differ from that found in mammalian cells.
To address this deficiency, new strains of yeast and fungi are being developed that produce posttranslational modifications that more closely resemble those found in native human proteins. Thus, there has been renewed interest in using yeast and fungal cells to express more complex proteins. However, due to the industry's focus on mammalian cell culture technology for such a long time, the fungal cell expression systems such as Trichoderma are not as well established as mammalian cell culture and therefore suffer from drawbacks when expressing mammalian proteins.
Thus, a need remains in the art for improved filamentous fungal cells, such as Trichoderma fungus cells, that can stably produce heterologous proteins, such as immunoglobulins, preferably at high levels of expression.